xenserver: Reduce number of xapi DB calls in plugin

xenserver: Reduce number of xapi DB calls in plugin

When there are lots of PIFs in a XenServer/XCP pool, for example when
there are many VLANs configured on the pool, operations such as
PIF.get_all and loops over all PIFs which include database operations,
are very inefficient when executed on a pool slave, and should be
avoided as much as possible. This patch reduces the number of database
calls in the update function of the openvswitch-cfg-update xapi plugin.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Acked-by: Dominic Curran <Dominic.curran@citrix.com>